In the present study energy efficient, economically scalable colloidal silver (Ag) nanoparticles were biosynthesized from marine brown seaweed Colpomenia sinuosa by green synthesis method. The marine macroscopic brown seaweed Colpomenia sinuosa was used in the experimental study for the biosynthesis of silver nanoparticle since they are rich in phytochemicals, bioactive compounds and secondary metabolites which has reducing agents that may be environmentally acceptable and eco-friendly. The biosynthesized silver nanoparticles from marine macroscopic brown seaweed were characterized by UV-vis spectroscopy which confirmed the surface plasmon resonance of silver nanoparticles, Fourier transform infrared (FT-IR) spectroscopy to identify the presence of various functional groups in biomolecules responsible for the bio reduction of Ag⁺ and capping/stabilization of silver nanoparticles. X-ray diffraction (XRD) to observe face center cubic (fcc) and crystalline nature of silver nanoparticles, thermogravimetric analysis (TGA) which revealed the thermal stability and purity of the silver nanoparticles. Particle size distribution and morphology were investigated by scanning electron microscope (SEM) which showed silver nanoparticles in the size range of 54-85 nm. The particle distribution under different nanometers was analyzed using transmission electron microscopy (TEM).